1. Field of the Invention
This invention relates to a method and an apparatus for measuring gas temperature and flow. In more specific aspect, the invention is a portable thermo-anemometer based on bimetallic sensing element.
2. Description of the Prior Art
Heretofore, many devices and systems are employed for measurement of temperature, in general, and measurement of ambient air temperature, in particular. These devices range from a simple household mercury thermometer to a technically sophisticated laboratory instrument, such as a high response two-dimensional thermocouple with a super sensitive D.C. milli-microvoltmeter system which this inventor used in his dissertation work.
Unfortunately, none of the existing devices is suitable for a homeowner in detecting and evaluating various heat losses which are commonly occuring in a residential unit. A low cost portable heat loss and draft detector will help in conserving fuel which is becoming more scarce and expensive.
Recently, infra-red detection devices have been used to map the exterior temperature profile or housing unit for heat loss detection. These devices are generally expensive and require a trained operator. Similar mapping of interior temperature profile is far more desirable in assessing the local heat loss through the walls or windows by conduction and, more importantly, in locating the cold air stream, draft, entering from the outside. Also information on the velocity of the stream is equally important. Infra-red detectors are not suited for this use. A small hand-held thermo-anemometer is a logical solution.
Obviously, a low cost portable thermometer, based on either a thermocouple or a thermistor with a simple circuit and a meter, can be easily constructed by anyone with an elementary knowledge of electronics. However, usefulness of such a device is doubtful because it can provide only limited sensitivity. Expensive components are necessary for detection and measurement of small voltage change in a thermocouple or of current change through a thermistor, which requires a stable power supply in addition.
For a device to be useful for heat loss and draft detection, it must be sensitive to the ambient air temperature drop of a fraction of a degree and, at the same time, responsive to the temperature change in order of a few seconds. Also the sensing element must have well-defined geometry for use as a temperature based anemometer. All commercially available units which meet these requirements are expensive scientific laboratory instruments and are not very handy.
Although bimetallic, or thermostatic, strips, in various forms, are widely used as a temperature sensing element in inexpensive thermometers and temperature control devices, no attempt had been made to use these elements for an extremely sensitive thermo-anemometer. In prior art devices, the temperature dependent deformation of fixed geometry bimetallic strip and the balance between this thermal deformation and the elastic restoring force of the metals were utilized. Additionally, in many control applications, the electro-conductivity of the metals was used as a current path. In time-delay switches, an external heating and heat capacity of bimetal were used. In these prior applications, high sensitivity was not the object.